19. (1) Save as hereinafter provided in this section, it shall be no defence in a prosecution under this Chapter to prove merely that the accused was ignorant of the nature, substance or quality of the drug in respect of which the offence has been committed or of the circumstances of its manufacture or import, or that a purchaser, having bought only for the purpose of test or analysis, has not been prejudiced by the sale. (2) For the purposes of section 18 a drug shall not be deemed to be misbranded or to be below standard quality only by reason of the fact that-(a) there has been added thereto some innocuous substance or ingredient because the same is required for the manufacture or preparation of the drug as an article of commerce in a state fit for carriage or consumption, and not to increase the bulk, weight or measure of the drug or to conceal its inferior quality or other defects; or(b) in the process of manufacture, preparation or conveyance some extraneous substance has unavoidably become intermixed with it: Provided that this clause shall not apply in relation to any sale or distribution of the drug occurring after the vendor or distributor became aware of such intermixture. (3) A person, not being the manufacturer of a drug or his agent for the distribution thereof, shall not be liable for a contravention of section 18 if he proves-(a) that he did not know, and could not with reasonable diligence have ascertained, that the drug in any way contravened the provisions of that section, and that the drug while in his possession remained in the same state as when he acquired it; or(b) that he acquired the drug from a person resident in Bangladesh under a written warranty in the prescribed form and signed by such person that the drug does not in any way contravene the provisions of section 18, and that the drug while in his possession remained in the same state as when he acquired it: Provided that a defence under clause (b) shall be open to a person only-(i) if he has, within seven days of the service on him of the summons, sent to the Inspector a copy of the warranty with a written notice stating that he intends to rely upon it and giving the name and address of the warrantor, and(ii) if he proves that he has, within the same period, sent written notice of such intention to the said warrantor.